I have a rule of thumb to have at least 3" ground clearance (4-fingers) when level. My planes are all in excess of 18 lbs. A slight bounce landing now and then should not kill my prop. We often land with heavy turbulence due to nearby trees.

Prop question:
Rule for changing to 3-bladed prop within same brand and have the same engine load:
go down 10% in diameter
go up 10% in pitch